Particle transport in pool scrubbing processes

Kunšek, Matic (author), Cizelj, Leon (mentor), Kljenak, Ivo (co-mentor)

During a hypothetical severe accident in a light water nuclear power plant, the reactor fuel could melt and there is a possibility, that some of the radioactive material could be released as particles to the surrounding area. The releases of the radioactive material can be reduced with the application of pool scrubbing. Pool scrubbing research falls under the severe accident field, more specifically the source term topic. During pool scrubbing, the release of contaminated gaseous mixture is filtered through a pool of liquid water. The mixture can be composed from different radioactive gases and vapors with particles present. The pool scrubbing region can be divided into three parts: inlet region, rise region and surface region. Each region is determined by its own characteristical phenomena. The inlet region can be a highly turbulent jet or a globule, the rise region consists of rising gas bubbles with particles and the bubbles merge with the air above the pool surface in the surface region. The scope of the present work is the description of the rise region on the local instantaneous scale with the use of computational fluid dynamics. The work is separated in three stages: modelling of single bubble decontamination, validation of the modelling of hydrodynamic phenomena within the used open-source solver and pool scrubbing simulation. In the first stage (single bubble decontamination), a numerical simulation of particle removal from single spherical bubbles is performed. From the calculated results, a subgrid model for bubble filtration rate is developed and implemented in the used numerical solver. In the second stage (solver validation) the numerical simulations of the bubbly and jet flows are performed and validated against experimental results. In the third part, numerical simulations of the pool scrubbing itself are performed. The results are then compared to experiments. In all stages, all phases are considered in Euler notation. Even though the presented bubble filtration model is relatively simple, it gives satisfactory calculated decontamination factors for comparison with experiments facilities in which mostly bubbly flow is present. For experiments where the jet region plays a major role, the comparison of simulation results with experiments shows a lesser agreement. Still, the used solver provides adequate results for the rise region of pool scrubbing and the discrepancies are mostly due to imperfections in modeling of the simulations hydrodynamic part.

• Language: English
• Keywords: pool scrubbing, subgrid model, decontamination, source term, severe accident
• Work type: Doctoral dissertation
• Typology: 2.08 - Doctoral Dissertation
• Organization: FMF - Faculty of Mathematics and Physics
• Year: 2023
• PID: 20.500.12556/RUL-151235
• COBISS.SI-ID: 167228419
• Publication date in RUL: 01.10.2023

Secondary language

• Language: Slovenian
• Title: Transport delcev pri procesih bazenske filtracije

Abstract

Med hipotetičnimi težkimi nesrečami v lahkovodnih jedrskih elektrarnah bi lahko prišlo do taljenja goriva, pri čemer obstaja verjetnost, da bi lahko prišlo do izpusta nekaj radioaktivnega materiala v obliki delcev v okoliški prostor. Omenjene izpuste se lahko blaži z uporabo bazenske filtracije (angl. pool scrubbing), kjer kontaminirani plini prehajajo skozi bazen kapljevite vode. Raziskave bazenske filtracije spadajo pod okrilje raziskav težkih nesreč (angl. severe accidents), podrobneje pod raziskave o izpustih radionuklidov (angl. source term). Plinasto zmes lahko sestavljajo radioaktivni plini in pare, v katerih so prisotni delci. Območje procesa bazenske filtracije lahko delimo na tri dele: območje vstopa, območje dviga in območje gladine. Vsako območje se imenuje po svojem glavnem karakterističnem pojavu. Območje vstopa je visoko turbulenten curek plina ali plinska polkrogla, območje dviga predstavljajo dvigajoči se plinski mehurčki, v območju gladine pa prihaja do združevanja mehurčkov nad gladino bazena. To delo se ukvarja z območjem dviga s popisom na trenutni lokali skali in uporabo računske dinamike tekočin. V predstavljenem delu je pristop razdeljen na tri korake: modeliranje filtracije mehurčka, validacija uporabljenega modela hidrodinamskih pojavov v sklopu odprtokodnega računalniškega programa in numerična simulacija same bazenske filtracije. Prvi korak (filtracija mehurčka) predstavljajo numerične simulacije, s katerimi popišemo krogelni prosto dvigajoči se mehurček, v katerem se nahajajo delci. Iz rezultatov pri različnih pogojih konstruiramo podmrežni model filtracije mehurčkov in ga implementiramo v uporabljeni program. V drugem koraku (validacija odprtokodnega programa) izvedemo numerične simulacije tako za mehurčkast režim kot za režim potopljenega curka in ju primerjamo z eksperimentalnimi rezultati. V tretjem koraku (bazenska filtracija) izvedemo večfazno numerično simulacijo bazenske filtracije ter rezultate primerjamo z eksperimentalnimi. V vseh korakih vse faze obravnavamo v Eulerjevem zapisu. Kljub temu, da je uporabljen model filtracije mehurčka razmeroma enostaven, daje primerjava izračunanih dekontaminacijskih faktorjev z eksperimenti, v katerih prevladuje mehurčkast tok, razmeroma dobre rezultate. Za eksperimente, ki izkazujejo izrazito območje vstopnega curka, so rezultati bistveno slabši. Razlike z eksperimentalnim dekontaminacijskim faktorjem v področju dviga pri procesih bazenske filtracije so večinoma posledice nepopolnosti modeliranja v hidro-dinamskem delu numerične simulacije.

• Keywords: bazenska filtracija, dekontaminacija, transport radionuklidov, težke nesreče